Relating energy demand
to degree days

Should any unforeseen event occur which causes unexpected excess energy consumption, the plot for that month (or week) will fall further above the target characteristic than usual.

Look at the illustrated example. For a degree-day value of a, the 'correct' or expected ration of energy was b but the actual quantity is was c. The extent of the deviation (c-b) is a measure of the unnaccountable lost energy.

Knowing the price per extra unit, you can evaluate the cash cost of the deviation and decide what if anything to do about it.